a) Field of the Invention
The present invention relates to a method of manufacturing an electrode base member and a reflecting member for a reflective (semi-transmissive) type liquid crystal display device including a film or the like as a base member, to an electrode base member and a reflecting member manufactured by the method, and to a liquid crystal display device including the electrode base member and the reflecting member.
b) Description of the Prior Art
In recent years, applications of liquid crystal display devices have rapidly widened and now include use in information appliances and the like owing to their characteristics such as low power consumption, low voltage operation, light weight, thinness and capability for color display.
In particular, a reflective simple matrix liquid crystal display device is drawing attention as suitable for a portable terminal device, owing to its low power consumption and lack of need for a backlight.
In this simple matrix liquid crystal display device, plural stripe-shaped transparent electrodes are formed on two base members, and the two base members are disposed in such a manner that the transparent electrodes of the two base members are perpendicular to each other. Liquid crystal is filled into a space between the two base members and pixel electrodes are formed at positions corresponding to each intersection point of two mutually perpendicular transparent electrodes. Thus, images can be displayed by controlling the liquid crystal.
A reflective liquid crystal display device includes a reflecting member for diffusing and reflecting external light for illumination. The reflecting member of the reflective liquid crystal display device is categorized into two types: one is an outward reflecting member type in which the reflecting member is formed on a plane opposite a liquid crystal layer of the base member, and the other is an inward reflecting member type in which the reflecting member is formed on a plane facing toward the liquid crystal layer of the base member.
Since the reflecting member is formed opposite the liquid crystal layer of the base member in the outward reflecting member type, the external light diffused and reflected by the reflecting member is first transmitted through the base member and then through the liquid crystal layer, and finally is emitted out of a display screen. In such devices, reflected light may cause parallax attributable to the thickness of the base member, resulting in defocused images such as double images or color-blending.
Since the reflecting member is formed facing the liquid crystal layer of the base member in the inward reflecting member type, the external light diffused and reflected by the reflecting member is transmitted through the liquid crystal layer without passing through the base member, and is then emitted at the display screen. Since the reflecting member and the liquid crystal layer are designed to exclude the influences on the reflected light by the thickness of the base member, parallax does not occur, whereby the defocused images are eliminated.
When a film is used as the base member, the film can be made thinner than a glass base. Accordingly, the parallax may be reduced to that extent. Nevertheless, when a color filter is included in the liquid crystal display device, one pixel is divided into three color dots, each dot having one-third the size of the pixel. In this case, the reflected light may be transmitted through untargeted dots more easily than in a monochrome liquid crystal display device. Thus, the influence of the film thickness becomes negligible.
Therefore, even if a film is used as the base member, the inward reflecting type should be adopted, particularly for a color liquid crystal display.
However, the liquid crystal display device of the inward reflecting member type tends to have a complex structure and, accordingly, manufacture thereof may be difficult.
When a film is used as the base member, the film tends to expand or contract under influences of heat or humidity and thus materials and process conditions are restricted.
The reflecting member has a rough surface in order to fulfill its function to diffuse and reflect the external light. Because this roughness may adversely affect driving of the liquid crystal, sophisticated planarization technology is specially required, which may raise manufacturing costs or reduce process yields.
Moreover, a metal layer formed as the reflecting member may be damaged in the course of subsequent chemical treatment performed in forming the transparent electrodes or the like. Furthermore, since a film is used as the base member, the film tends to expand or contract under the influences of heat and humidity, thus restricting the materials and manufacturing conditions for the reflecting member and the transparent electrodes. Accordingly, it becomes difficult to manufacture an electrode base member of a liquid crystal display device which corresponds to the design demands.
In addition, the following problems are observed in the manufacture of an electrode base member for a liquid crystal display device of a semi-transmissive type, which is designed to use both a backlight and reflected external light as light sources, by means of forming a reflecting member or the like directly on a film. In this case, a reflecting member made of a metal layer is first formed covering the entire surface of the film. Then, portions of the metal layer, corresponding to the pixel of the transparent electrodes to be formed in a subsequent step, need to be partially removed with high accuracy in order to allow the back light to be transmitted therethrough. The portions to be removed have areas smaller than the pixels of the transparent electrodes.
The ITO serving as the transparent electrodes is formed in such a manner that the portions of the transparent electrodes to be the pixels overlap the areas in which the reflecting member made of the metal layer is removed. A resist layer is formed on the ITO while aligning the resist layer and the ITO with good accuracy. After exposing and developing the resist layer the ITO is removed. This process requires a high degree of alignment accuracy.
However, a film made of plastic is expanded easily just by water washing, for example, and, conversely, the film contracts when dried. In addition, such expansion or contraction is not immediately stabilized, and requires a long period of time to stabilize. In other words, for example, once the film is contracted, dimensions of the patterns formed on the film show that contraction for a long period of time. In such a situation, reproducibility of the above-mentioned alignment is difficult to obtain.
Therefore, it is difficult to manufacture the electrode base member for the semi-transmissive liquid crystal display device by means of forming the reflecting member or the transparent electrodes directly on the film.
Another proposed method for forming the reflecting member involves creating roughness on a surface of a resin layer, such as a resist layer, by photolithography using a photomask and forming a metal layer to reflect light on this rough surface.
However, in forming roughness on the surface of a resin layer, such as the resist layer, by photolithography, simple repetition of planar patterns on the reflecting member may cause the reflecting member to function as a diffraction grating. Accordingly, when the liquid crystal display screen is viewed, defects such as iridescence or so-called moire fringes may occur due to subtle errors in positioning relative to other repetitive patterns of wiring, black matrices or the like. For this reason, simple duplicative patterns such as pixel patterns cannot be used in designing the photomask for forming the rough patterns, and extremely vexatious and complicated designing is required. Thus, it is not easy to form reflecting members having proper diffusing capacity by normal photolithographic methods.
Other methods for forming roughness on a resin surface of resin are also known, such as: incorporation of diffusing particles of a different material into the resin (Japanese Patent Publication Hei 4-267220); use of phase separation of two different types of resin upon curing (Japanese Patent Publication 2000-193807); and forming roughness by curing a surface portion of the resin on its surface side and exposing and baking while leaving its interior uncured (Japanese Patent Publication 2000-171792).
In forming a reflecting member directly on a plastic film having a thickness from 50 to 200 μm by use of the above-mentioned conventional methods, stress upon curing of the resin may form cambers in the plastic film.
In addition, the reflecting member manufactured in accordance with any of the foregoing manufacturing methods would have a composition of materials that are virtually different in refractive indices on opposing sides of a boundary such as an interface of the resin and the particles or an interface of separated phases. When the rough face of the reflecting member is formed on the side of the liquid crystal layer opposite the electrode base member, incident light is polarized by a polarizing plate, subsequently transmitted through the interface of the materials of the different refractive indices via the liquid crystal layer, and then reflected by the metal layer. In this event, the polarized incident light and the light dispersed by the metal layer may show depolarization due to reflection at the interface of the materials of the different refractive indices whereby the degree of polarization of the light is reduced. Accordingly, when a reflecting member composed of materials having different refractive indices is used in a liquid crystal display device of the inward reflecting member type, it is likely that the contrast ratio of the liquid crystal display screen will be reduced.